Re: [PATCH] fix a race between /proc/lock_stat and module unloading

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 02, 2015 at 11:54:13AM +0200, Jerome Marchand wrote:
> 
> I guess I jumped to conclusion here and my explanation is wrong. However
> there is still a bug which occurs when the kernel tries to access
> class->name is seq_stats:
> 
> [   43.533732] BUG: unable to handle kernel paging request at
> ffffffffa03181ce
> [   43.534006] IP: [<ffffffff8142b489>] strnlen+0x9/0x50

Does something like so cure things?

---
 kernel/locking/lockdep.c      |  3 ++-
 kernel/locking/lockdep_proc.c | 22 +++++++++++++++++-----
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a0831e1b99f4..50ed2685f937 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3900,7 +3900,8 @@ static void zap_class(struct lock_class *class)
 	list_del_rcu(&class->hash_entry);
 	list_del_rcu(&class->lock_entry);
 
-	class->key = NULL;
+	WRITE_ONCE(class->key, NULL);
+	WRITE_ONCE(class->name, NULL);
 }
 
 static inline int within(const void *addr, void *start, unsigned long size)
diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
index ef43ac4bafb5..03946779eacc 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -426,10 +426,12 @@ static void seq_lock_time(struct seq_file *m, struct lock_time *lt)
 
 static void seq_stats(struct seq_file *m, struct lock_stat_data *data)
 {
-	char name[39];
-	struct lock_class *class;
+	struct lockdep_subclass_key *ckey;
 	struct lock_class_stats *stats;
+	struct lock_class *class;
 	int i, namelen;
+	char name[39];
+	char *cname;
 
 	class = data->class;
 	stats = &data->stats;
@@ -440,15 +442,25 @@ static void seq_stats(struct seq_file *m, struct lock_stat_data *data)
 	if (class->subclass)
 		namelen -= 2;
 
-	if (!class->name) {
+	rcu_read_lock_sched();
+	cname = rcu_dereference_sched(class->name);
+	ckey  = rcu_dereference_sched(class->key);
+
+	if (!cname && !ckey) {
+		rcu_read_unlock_sched();
+		return;
+
+	} else if (!cname) {
 		char str[KSYM_NAME_LEN];
 		const char *key_name;
 
-		key_name = __get_key_name(class->key, str);
+		key_name = __get_key_name(ckey, str);
 		snprintf(name, namelen, "%s", key_name);
 	} else {
-		snprintf(name, namelen, "%s", class->name);
+		snprintf(name, namelen, "%s", cname);
 	}
+	rcu_read_unlock_sched();
+
 	namelen = strlen(name);
 	if (class->name_version > 1) {
 		snprintf(name+namelen, 3, "#%d", class->name_version);